The carrier boards are used in conjunction with the G4 Series Opto-22 Industry Standard Digital I/O Modules. The modules are available as DC Input, DC Output, DC Reed Relay Output, AC Input and AC Output models.

A solid state relay (SSR) is an electronic switching device in which a small control signal controls a larger load current or voltage. The relay may be designed to switch either AC or DC to the load. It serves the same function as an electromechanical relay, but has no moving parts.